It is known, in principle, that a step-up converter and a step-down converter can be combined between two DC-voltage intermediate circuits to form a half-bridge, which is connected between the poles of the DC-voltage intermediate circuit for a higher intermediate circuit voltage and the center point of which is connected via an inductor to a pole of the DC-voltage intermediate circuit for the lower intermediate circuit voltage. In the case of such a half-bridge, the step-down converter freewheeling diode is connected in parallel with the step-up converter switch and the step-up converter freewheeling diode is connected in parallel with the step-down converter switch. In order to be able to keep the inductance and hence the installation size of the inductor connected to the center point of the half-bridge, the switch in the respective step-up converter operation or step-down converter operation of the half-bridge must be clocked at a high frequency (usually several ten kHz). In order to realize this with low switching losses, MOSFET semiconductor switches, which cause comparatively low switching losses when clocked at high frequency, are preferred. However, MOSFET semiconductor switches have parasitic internal diodes which are referred to as body diodes. These are connected in parallel with the freewheeling diodes of the semiconductor bridge and can therefore, in principle, become conducting under the same boundary conditions as the freewheeling diodes. However, they are not suitable for rapid switching and, in the case of rapid switching, would cause high switching losses, poor EMC behaviour and, in some circumstances, would even cause the circuit to be destroyed.
A reactive power-capable inverter having two parallel half-bridges connected between a first connection and a second connection is known from U.S. Pat. No. 6,847,196 B2. Each half-bridge has two parallel conduction paths. In each of the conduction paths a MOSFET as a switching element is connected in series with a freewheeling diode. One switching element in one of the conduction paths and one freewheeling diode in the other of the conduction paths are connected to the first and to the second connection, respectively. The center points of the two conduction paths are connected to one another via two small inductors. The connection point of the two inductors is connected via a further inductor to one alternating-current connection. The other alternating-current connection is connected to a corresponding connection point of the other half-bridge. By alternating operation of the two half-bridges as step-down converters, successive half-waves of an alternating current are formed, which is output at the alternating-current connections. The respective non-current-forming half-bridge is switched through. The alternatively possible step-up converter operation of the half-bridges makes it possible to reverse the flow of energy and hence, for example, also to output reactive power at the alternating-current connections. During operation of the known inverter, commutation of the current from the respectively clocked MOSFET to the body diode of the MOSFET in the other conduction branch of the same half-bridge is prevented by the inductive voltage division of the inductors, with the result that the current exclusively commutates onto the freewheeling diode connected in series with the clocked MOSFET in the same conduction path. The known inverter is comparatively elaborate owing to three inductors in total per half-bridge. It also has a total of four comparatively expensive MOSFETs.
A reactive power-capable inverter is known from U.S. 2011/0013438 A1, in the case of which both MOSFET semiconductor switches and IGBT semiconductor switches are used. The known inverter is of the NPC (neutral point clamped) circuit type with freewheeling diodes for outputting reactive power. U.S. 2011/0013438 A1 also describes circuits having half-bridges, in the case of which conduction paths populated with MOSFET semiconductor switches are decoupled by inductors. In the case of inverters with NPC circuit which have no more than two MOSFET semiconductor switches and two IGBT semiconductor switches, no inductors which decouple any conduction paths are provided, however.
The combination of a step-down converter circuit and a commutator to form a photovoltaic inverter is known from EP 2 421 135 A2. The step-down converter circuit forms an incoming direct current in half-waves, which are converted into an alternating current by means of the commutator circuit. The step-down converter circuit has two partial step-down converters which are designed and arranged so as to be mirror-symmetrical about a center point apart from the forward directions of their diodes and the blocking directions of their switches. Even if the step-down converter circuit enables a phase shift of the current with respect to the voltage in each half-wave, it is not unconditionally reactive power-capable. It does not enable different mathematical signs for current and voltage at the alternating-current output of the photovoltaic inverter and is therefore only able to output reactive power at a high distortion factor.
A circuit is known from U.S. Pat. No. 5,107,151 A, in the case of which two inputs are connected via a half-bridge to an output. The half-bridge has two conduction paths between the two inputs. In each of the conduction paths one switching element is connected in series with a freewheeling diode. In this case, the switching element in the one conduction path is connected to the one input and the switching element in the other conduction path is connected to the other input. The opposite applies in the case of the freewheeling diodes. Center points of the conduction paths are connected to one another via an inductor. The center point of the second conduction path is connected via a further inductor to an output of the circuit. Two of such circuits can be combined to form a full-bridge in order to drive an electric motor, wherein only one further inductor is provided in addition to the inductors between the center points of the conduction paths of the two bridges. During operation of the known circuit, in the case of each half-bridge only one of the switching elements, which is used for pulse-width modulation, is clocked at a high frequency, while the other switching element, which is used for commutation, is clocked at a low frequency. In the case of the different clock rates of the switching elements, the switching element which is clocked at a low frequency is designed as an IGBT, in contrast to the switching element which is clocked at a high frequency, which is a MOSFET. U.S. Pat. No. 5,107,151 A does not describe a bidirectional converter or bidirectional inverter having switching elements of different design in the half-bridges.
Thus, there is still a need for a bidirectional converter and reactive power-capable inverters based thereon, which have a simple and inexpensive design while still having a high practical efficiency.